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United States Patent |
5,147,736
|
Lapp
|
September 15, 1992
|
Metal/air fuel cell with electrolyte flow equalization manifold
Abstract
A manifold system is described for equalizing electrolyte flow to a
plurality of metal/air cells of a fuel cell assembly. The fuel cell
comprises: (a) a housing, (b) a plurality of metal/air cells disposed
vertically in the housing, (c) air injection means for flowing oxidizing
air between the metal/air cells, (d) an electrolyte storage tank, (e) a
recirculation loop for continuously recirculating electrolyte from the
storage tank through the metal/air cells, (f) an electrolyte inlet
manifold forming part of said recirculation loop, said manifold comprising
a large manifold tube extending horizontally beneath a plurality of said
metal/air cells and a plurality of small feeder tubes extending
horizontally, laterally from said large tube, each small feeder tube
extending across beneath a single metal/air cell and flow connecting to
the bottom of the cell, said large tube having a diameter sufficiently
greater than the diameter of the small feeder tubes such that the total
combined flow of all of the small feeder tubes does not cause a
significant pressure drop in the large manifold tube. Each feeder tube has
a length and diemeter to provide a friction pressure drop therethrough
which is sufficiently high that the static pressure head difference due to
elevation at the cell inlet between the lowest and highest cells in an
inclined stack is very small compared to the pressure drop across each
individual small feeder tube.
Inventors:
|
Lapp; Steven P. (Sydenham, CA)
|
Assignee:
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Alcan International Limited (Montreal, CA)
|
Appl. No.:
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773510 |
Filed:
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October 9, 1991 |
Current U.S. Class: |
429/27; 429/14; 429/39 |
Intern'l Class: |
H01M 008/04; H01M 004/00 |
Field of Search: |
429/14,39,35,27
|
References Cited
U.S. Patent Documents
3520731 | Jul., 1970 | Rightmire et al. | 429/14.
|
3666561 | May., 1972 | Chiku | 136/86.
|
4210512 | Jul., 1980 | Lawrance et al. | 429/39.
|
4520080 | May., 1985 | Hashimoto | 429/18.
|
4910102 | Mar., 1990 | Rao et al. | 429/51.
|
4910104 | Mar., 1990 | Rao et al. | 429/67.
|
Primary Examiner: Maples; John S.
Attorney, Agent or Firm: Cooper & Dunham
Claims
I claim:
1. A fuel cell assembly comprising:
(a) a housing,
(b) a plurality of metal/air cells disposed vertically in the housing,
(c) air injection means for flowing oxidized air between the metal/air
cells,
(d) an electrolyte storage tank,
(e) a recirculation loop for continuously recirculating electrolyte from
the storage tank through the metal/air cells,
(f) an electrolyte inlet manifold forming part of said recirculation loop,
said manifold comprising a large manifold tube extending horizontally
beneath a plurality of said metal/air cells and a plurality of small
feeder tubes extending horizontally, laterally from said large tube, each
small feeder tube extending across beneath a single metal/air cell and
flow connecting to the bottom of the cell, said large tube having a
diameter sufficiently greater than the diameter of the small feeder tubes
such that the total combined flow of all of the small feeder tubes does
not cause a significant pressure drop in the large manifold tube.
2. A fuel cell according to claim 1 wherein each small feeder tube has a
length and diameter to provide a friction pressure drop therethrough which
is sufficiently high that the static pressure head difference due to
elevation at the cell inlet between the lowest and highest cells in an
inclined cell stack is very small compared to the friction pressure drop
across each individual small feeder tube.
3. A fuel cell according to claim 1 wherein the metal/air cells are
aluminum-air cells.
4. A fuel cell according to claim 3 wherein the aluminum comprises aluminum
anode plates.
5. A fuel cell according to claim 4 wherein the anode plates are in the
form of a refuelling anode assembly comprising:
(a) a non-conducting plastic top cover plate for fuel cells,
(b) a plurality of aluminum or aluminum alloy anode plates extending
downwardly from said cover plate in vertical, equally spaced arrangement,
(c) a U-shaped caustic-resistant plastic cap sealingly mounted on the top
edge of each anode plate,
(d) electrically conductive fasteners extending through said non-conducting
cover plate and into said anode plates to securely hold the anode plates
snugly against said cover plate, and
(e) electrical conducting means on the top of the cover plate connected to
said conductive fasteners.
6. An aluminum-air fuel cell assembly comprising:
(a) a fuel cell assembly housing having closed side and bottom walls and an
open top,
(b) a plurality of vertical, equally spaced cells in said housing
comprising caustic electrolyte cells between air cathodes and air gaps
between the faces of the air cathodes remote from said electrolyte,
(c) means for moving air through said air gaps and for moving caustic
electrolyte through said electrolyte cells, said means for moving caustic
electrolyte comprising a recirculation loop for continuously recirculating
electrolyte from a storage and through the metal/air cells by way of a
manifold including a large manifold tube extending horizontally beneath a
plurality of said metal/air cells and a plurality of small feeder tubes
extending horizontally, laterally from said large tube, each small feeder
tube extending across beneath a single metal/air cell and flow connecting
to the bottom of the cell, said large tube having a diameter sufficiently
greater than the diameter of the small feeder tubes such that the total
combined flow of all of the small feeder tubes does not cause a
significant pressure drop in the large manifold tube,
(d) a refuelling anode assembly mounted on said housing open top, said
anode assembly comprising a non-conducting plastic top cover plate, a
plurality of aluminum or aluminum alloy anode plates extending downwardly
from said cover plate in vertical, equally spaced arrangement and into
corresponding housing electrolyte cells, electrical conducting means
extending through said cover plate and connecting to said anode plates.
Description
BACKGROUND OF THE INVENTION
The invention relates to metal/air fuel cells, and particularly to an
electrolyte flow equalization manifold for such fuel cells having
recirculating electrolyte
Metal/air fuel cells or batteries produce electricity by the
electro-chemical coupling of a reactive metallic anode to an air cathode
through a suitable electrolyte in a cell. The air cathode is typically a
sheet-like member, having opposite surfaces respectively exposed to air
and to the aqueous electrolyte of the cell. During cell operation, oxygen
is reduced within the cathode while metal of the anode is oxidized,
providing a usable electric current flow through external circuitry
connected between the anode and cathode. The air cathode must be permeable
to air but substantially impermeable to aqueous electrolyte, and must
incorporate an electrically conductive element to which the external
circuitry can be connected. Present-day commercial air cathodes are
commonly constituted of active carbon (with or without an added
dissociation-promoting catalyst) in association with a finely divided
hydrophobic polymeric material and incorporating a metal screen as the
conductive element. A variety of anode metals have been used or proposed;
among them, zinc, alloys of aluminum and alloys of magnesium are
considered especially advantageous for particular applications, owing to
their low cost, light weight and ability to function as anodes in
metal/air fuel cells using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a
sheet-like air cathode having one surface exposed to the electrolyte and
the other surface exposed to air, and an aluminum alloy anode member (e.g.
a flat plate) immersed in the electrolyte in facing spaced relation to the
first-mentioned cathode surface. A typical fuel cell unit or battery
comprises a plurality of such cells.
Aqueous electrolytes for metal/air fuel cells consist of two basic types,
namely a neutral-pH electrolyte and a highly alkaline electrolyte. The
neutral-pH electrolyte usually contains halide salts and, because of its
relatively low electrical conductivity and the virtual insolubility of
aluminum therein, is used for relatively low power applications. The
highly alkaline electrolyte usually consists of NaOH or KOH solution, and
yields a higher cell voltage than the neutral electrolyte.
In alkaline electrolytes, the cell discharge reaction may be written:
4Al+3O.sub.3 +6H.sub.2 O+4 KOH.fwdarw.4Al(OH)hd 4+K.sup.+ (liquid
solution),
followed, after the dissolved potassium (or sodium) aluminate exceeds
saturation level, by:
4Al(OH).sub.4 +4K.sup.+ .fwdarw.4Al(OH).sub.3 (solid)+4KOH
In addition to the above oxygen-reducing reactions, there is also an
undesirable, non-beneficial reaction of aluminum in both types of
electrolyte to form hydrogen, as follows:
2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2 (gas)
Metal/air fuel cells are of particular interest as a fuel source for
motorized vehicles and when they are used for this purpose they must be
capable of being operated at an incline of at least 10.degree. to the
horizontal.
Typically, the problem of feeding a plurality of fuel cells simultaneously
has been solved by creating a fluid-tight stack through which fluid can be
forced such that the inclination of the stack of cells does not affect
fluid flow. Such an arrangement leads to refuelling difficulties as the
stack must be opened for refuelling which breaks the fluid-tight seals. A
basic goal in the refuelling of aluminum/air fuel cells is that the
refuelling must not require the use of fluid-tight pressure seals.
U.S. Pat. No. 3,666,561 issued May 30, 1972 describes an electrolyte
recirculating battery having a plurality of cells in which electrolyte is
flowed in through a main manifold tube, then up through individual tubes
and out of the fuel cells through individual discharge tubes. Short
connectors are used between the manifold tube and the individual cells.
U.S. Pat. No. 4,520,080 issued May 28, 1985 shows an arrangement of small
tubes between fuel cells for absorbing small shunt electrical currents.
However, this does not apply to a typical metal/air fuel cell, but relates
to a design with separate cathode and anode electrolytes.
U.S. Pat. No. 4,910,102 issued Mar. 20, 1990 describes an electrolyte flow
manifold for a metal/air cell in which the manifold arrangement is at the
top of the cells and the flow is from the top downwardly through each
cell.
None of the above designs is suitable for simple refuelling by replacement
of anodes.
It is the object of the present invention to develop a metal/air fuel cell
capable of being operated in an inclined position while maintaining
substantially equal electrolyte flow to individual cells and providing
shunt circuit protection.
SUMMARY OF THE INVENTION
The present invention in its broadest aspect relates to a metal/air fuel
cell assembly comprising:
(a) a fuel cell assembly housing,
(b) a plurality of metal/air cells disposed vertically in the housing,
(c) air injection means for flowing air between the metal/air cells,
(d) an electrolyte storage tank,
(e) a recirculation loop for continuously recirculating electrolyte from
the storage tank through the metal/air cells,
(f) an electrolyte inlet manifold forming part of said recirculation loop,
said manifold comprising a large manifold tube extending horizontally
beneath a plurality of said metal/air cells and a plurality of small
feeder tubes extending horizontally, laterally from said large tube, each
small feeder tube extending across beneath a single metal/air cell and
flow connecting to the bottom of the cell, said large tube having a
diameter sufficiently greater than the diameter of the small feeder tubes
such that the total combined flow of all of the small feeder tubes does
not cause a significant pressure drop in the large manifold tube.
Each small feeder tube has a length and diameter such as to provide a
friction pressure drop therethrough which is sufficiently high that the
static pressure head difference due to elevation between the lowest and
highest cells in an inclined stack is very small compared to the friction
pressure drop across each individual small feeder tube. The cell stack is
designed to continue operating at roll inclinations of up to 10 degrees.
In a typical commercial operation, a large manifold tube extends
horizontally across beneath 14 metal/air cells with 14 small feeder tubes
extending horizontally, laterally from the manifold tube. A typical
commercial manifold tube may have a diameter in the order of 15 to 30 mm,
while the corresponding small feeder tubes may have diameters in the order
of 2 to 5 mm and lengths in the order of 20 to 100 cm. By placing each
small feeder tube horizontally directly beneath the fuel cell which it is
feeding, there is a significant reduction in the shunt currents from cell
to cell.
When using a feed manifold system according to the present invention, it is
possible to use a refuelling arrangement such as that described in U.S.
Ser. No. 774,185, Steven P. Lapp, "Refuelling Anode Assembly for
Aluminum-Air Cells", (filed simultaneously herewith), incorporated herein
by reference. Thus, the fuel cell housing may be of an open top design and
a refuelling anode assembly can simply be set in place in the housing with
only a sponge gasket. It is, therefore, an important feature of the
present invention that the need for fluid-tight cell top seals is
eliminated and the open tops of the cells are available for refuelling
without disassembling the fuel cell stack.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features of the present invention will become apparent to those
skilled in the art to which the present invention relates from reading the
following specification with reference to the accompanying drawings, in
which:
FIG. 1 is a schematic flow diagram illustrating a system within which this
invention can be used;
FIG. 2 is an end elevation in partial section showing a fuel cell
incorporating this invention;
FIG. 3 is a side elevation in partial section showing the fuel cell of FIG.
2;
FIG. 4 is a top plan view of a refuelling rack; and
FIG. 5 is a partial sectional view showing the electrolyte feed arrangement
of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is particularly useful in an electrolyte circulation
system for an aluminum/air fuel cell.
A typical flow sheet for a system to which the present invention can be
applied is shown in FIG. 1. This shows a main fuel cell case or housing 10
containing aluminum/air fuel cell stacks 11. Alkaline electrolyte, e.g. a
solution of NaOH or KOH, is stored in storage tank 12 and is pumped via
inlet line 13 and pump 14 into the bottom of the aluminum/air cells. The
used electrolyte is returned via electrolyte return line 15 and recycle
pump 16. Air is fed to the fuel cell stacks 11 by an air feed pump 19.
The fuel cell generates hydrogen and some of this is trapped in the
electrolyte in the form of very small bubbles. A mixture of electrolyte
and hydrogen discharges through top outlet 17 into an electrolyte/gas
separator or degassing vessel 18. The electrolyte is largely degassed in
this vessel with the separating hydrogen passing upwardly through an open
top. It proceeds through outlet 20 and into a filter assembly 21 for
removing caustic mist and caustic droplets with hydrogen and air
substantially free of caustic being discharged to the atmosphere through
discharge outlet 22.
The electrolyte in the degassing vessel 18 continues to contain some
residual hydrogen in the form of very small bubbles and this is carried
back to reservoir 12 through return line 15. In the reservoir, the
hydrogen gradually separates from the liquid. In order to keep the
concentration of hydrogen in the space above the electrolyte in storage
tank 12 below about 2% by volume, purge air is pumped into the top of tank
12 by way of air pump 23 and air line 24. This purge air dilutes and
collects hydrogen from tank 12 and this mixture then passes via purge line
25 back to the degassing vessel 18.
This degassing vessel 18 is also bathed in a stream of air originating from
air feed 19 and exhausting from the aluminum/air cells 11 and the air
which is discharged through outlet 20 is a mixture of the air exhausting
from the aluminum/air cells 11 and the purge air from line 25.
The electrolyte flow system of the present invention will now be described
in greater detail with reference to FIGS. 2 to 5. As can be seen from
FIGS. 2 and 3, the fuel cell housing 10 contains 4 stacks of aluminum/air
cells with each stack containing 14 such aluminum/air cells thereby
providing a total of 56 cells in the complete fuel cell assembly. In this
particular unit, each anode has an area of approximately 950 cm.sup.2 and
the electrolyte circulates at a rate of about 25 1/min, giving a total
power output of about 8 Kw.
The fuel cell housing 10 includes a bottom wall 30, side walls 31, end
walls 33 and an open top. These are made from a chemical resistant plastic
material, such as polyvinylchloride or polyphenylene oxide. The
aluminum/air cells 35 are each made up of cathode walls 36 with air gaps
37 therebetween for the passage of oxidizing air. The cathode walls 36
also form therebetween electrolyte passages 38 through which electrolyte
moves from bottom to top. Each of these electrolyte passages 38 contains
an aluminum anode 39 which is mounted at the top end thereof to top wall
32 with an electrical connector 65 passing from each anode 39 up through
the top wall 32 and connecting to an anode busbar 40. The electrical
connection to the cathodes is made by way of cathode connector tabs 41.
The anodes 39 and top wall 32 are preferably in the form of a refuelling
anode assembly as described in U.S. Ser. No. 774,185, Steven P. Lapp,
"Refuelling Anode Assembly For Aluminum-Air Fuel Cells" (filed
simultaneously herewith), incorporated herein by reference. Each
refuelling assembly is intended to fuel one stack of aluminum-air cells
and thus comprises fourteen anode plates 39 connected to a top wall 32 by
way of conductive screws 65. Mounted on the top edge of each anode plate
39 is a U-shaped caustic-resistant plastic cap 66 to prevent any of the
electrolyte from coming in contact with the top edge of the anode. Each
refuelling assembly is inserted into a cell stack through the open top of
housing 10 with the anode plates 39 extending into the cells and the top
wall 32 resting on a sponge gasket 67. This provides a sufficient fluid
seal for the operation of the system.
The electrolyte being pumped from the reservoir 12 enters the fuel cell via
inlet 42 and it travels along large manifold tube 43 in the bottom region
of the housing. Extending laterally from this manifold tube 43 are a
series of small feeder tubes 44 with one of these flow tubes being used
for each aluminum/air cell as can be seen from FIG. 5. The electrolyte
enters each aluminum/air cell by way of individual flow passages 45.
As can be seen from FIGS. 2 and 5, each small feeder tube 44 extends across
directly beneath each fuel cell. Thus, the manifold tube 43 is adjacent
the inner corners of the cells in a stack and each flow connector 45 is at
an opposite bottom corner of each cell with the small feeder tube 44
extending therebetween.
As shown in FIG. 2, the used electrolyte and hydrogen gas discharge through
top outlet 17 and into electrolyte/gas separator or degassing vessel 18.
The numeral 47 represents a discharge outlet for a mixture of air,
hydrogen, caustic mist and caustic particles rising from the degassing
vessel 18. This passes upwardly and then through an exhaust filter 21
mounted on top of the fuel cell housing 10.
This filter 21 includes side walls 57 and 59, an intermediate wall 58, a
bottom wall 60 between walls 58 and 59 and an inclined top wall 61. The
exhaust gases enter between walls 57 and 58 and the clean air and gas
being discharged exits through outlets 22. The inlet portion includes a
filter 63 made of pads of 3M Scotchbrite.RTM. material for filtering out
caustic droplets and an automotive air intake paper filter 64 for removing
any fine mist. The gases being discharged through outlet 22 contain little
or no caustic mist or droplets.
From the above description of a preferred embodiment of the invention,
those skilled in the art will perceive improvements, changes and
modifications. Such improvements, changes and modifications within the
skill of the art are intended to be covered by the appended claims.
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